Honda C90 - carburettor - how it works

31st Dec 2019
Tags: carburettor

When I was trying to find out what I would need to do to get my bike to run I  soon discovered I would need to clean the carburettor in case it had become clogged up with old fuel.

Despite having watched various videos of people dismantling and cleaning them I  had no idea what all the strangely named parts did and began to agonise (unnecessarily as it turned out) about how complicated it seemed.  Eventually I just had to get on with it and take it apart and, needless to say, it all went back together without any drama.

After cleaning the carburettor the bike did run but didn’t idle well and was hard to start.   There was not much chance of finding out what was causing these problems, or indeed of even being sure it was the carburettor that was at fault, without first trying to understand at least the basics of how it actually worked.

This will be the first of a series of posts where I revisit some of the mechanical work I did on the engine, nearly all of which I did by blindly following instructions rather than understanding what I was doing or why it was necessary.

The next few posts will be my attempt to summarise what I have learned about carburettors from a combination of reading and taking things apart and staring at them.

For C90 owners, I should note that the particular model carburettor used on my 1978 C90 – a Keihin PB25A –  was only installed on the Z2 and ZZ models made in the late 1970s/early 1980s.   The carbs used on the later cubs are similar in design but there is one important difference (which effects how the carburettor is adjusted) and this is explained in a later post.

The (pre-clean) insides of my carbuerttor

How a carburettor works

As the piston in the engine moves downwards, air is sucked into the carburettor and is forced through a narrow constriction in the carburettor bore (called a venturi) and this causes the air to speed up and the pressure in this section to be reduced.

The fuel, which is held in a bowl at the base of the carburettor, is pushed by atmospheric pressure into the low pressure area in the venturi where it mixes with the air on the way through to the cylinder.  The air fuel mixture in the cylinder is then compressed by the now ascending piston and ignited, restarting the cycle and driving the engine.


That’s it! This is essentially all the carburettor does. But what is going on? With apologies in advance to any scientists who might be reading this, here is a rough explanation:

Venturi effect

The carburettor works because of the Venturi effect (named after Giovanni Venturi). This is the name for the reduction in pressure that occurs when a fluid (i.e. a liquid or a gas) flows through a constricted section in a pipe.

As the air passes through the narrowest point it has to speed up to get the same volume of air through the narrower gap and, as it accelerates, the pressure drops.

Luckily we don’t need to understand the mathematical relationship between air speed and pressure, we just need to know that fast moving air has lower pressure.

One way to think about what is happening is that the air is being stretched so that it contains fewer molecules per unit of volume and is therefore less dense, creating less pressure.

The relationship between fluid velocity and pressure was first observed by 18th C scientist called Daniel Bernoulli.  Here is a proper scientific explanation for those that might be interested.

One of the good folks over at the excellent Vintage Honda Twins forum uncovered this tremendous video of a home made transparent carb that shows the venturi effect in action:

The whole video is worth a watch, brilliant stuff.

Armed with this rudimentary understanding of fluid dynamics we can try and understand what the various parts of the carburettor are for.

PB25A schematic

As mentioned above, we will be looking at the carburettor that came with my c90 z2, a PB25A. It was made by Keihin, who have been making carburettors for Honda since the late 1950s, and is one of several designs used on the c90 over the years.

here is a schematic of the carb:

Keihin PB25A carburettor (1978 Honda C90)

it is not as complicated as it first appears, honest.  We look at what all the bits do in the next post



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